The present invention pertains to improvements in the field of liquid mirrors. More particularly, the invention relates to a method of treating a liquid gallium or gallium alloy surface for prolonged use as a liquid mirror.
Liquid mirrors are alternative solutions to solid metal-coated glass mirrors in optical instruments such as telescopes. They are based on the principle that the surface of a liquid metal rotating in a gravitational field takes the shape of a parabola, providing excellent reflective surface qualities at low costs. For a given diameter, the cost of a mercury liquid mirror is almost two orders of magnitude less than the cost of a glass mirror. However, mercury liquid mirrors suffer from a major limitation: they cannot be tilted and can only observe the zenith, thus greatly restricting their usefulness. In addition, mercury is a toxic substance.
Gallium and gallium alloys such as gallium-indium alloys have been used as non-toxic substitutes for mercury in liquid mirrors. They have the advantage of remaining in the liquid phase throughout a wide temperature range and have a very low vapor pressure at atmospheric pressure. However, a problem still remains in that there is a drastic oxidation which takes place at the gallium-air interface. A gallic oxide phase, Ga2O3, grows rapidly and produces a rough surface with large defects that diffracts light and destroys the reflection properties of the gallium or gallium alloy. The surface becomes rapidly matte. Since the mirror is normally used in air, and therefore in the presence of water and oxygen, the oxidation of gallium is unavoidable.
It is therefore an object of the present invention to overcome the above drawback and to provide a method of treating a liquid gallium or gallium alloy surface for prolonged use as a liquid mirror.
In accordance with the present invention, there is thus provided a method of treating a liquid gallium or gallium alloy surface, which comprises the steps of:
a) contacting the surface of liquid gallium or gallium alloy with an aqueous solution of a halogenic acid to cause dissolution of any gallium oxide present on the surface, thereby obtaining an oxide-free liquid gallium or gallium alloy surface covered with a layer of the acid solution;
b) adding to the acid solution an aqueous solution of a surfactant present in an amount to form a single bimolecular layer of surfactant at an interface between the liquid gallium or gallium alloy and water; and
c) allowing a uniform passivating oxide layer to form on the oxide-free liquid gallium or gallium alloy surface, the passivating oxide layer having surface irregularities smaller than 40 nm.
Applicants have found quite unexpectedly that by forming on an oxide-free liquid gallium or gallium alloy surface a uniform passivating oxide layer having surface irregularities smaller than 40 nm, such a passivating oxide layer protects the liquid gallium or gallium alloy surface against undesirable oxidation and maintains the reflection properties of the surface for a long period of time. A passivating oxide layer having surface irregularities larger than 40 nm adversely affect the reflection properties of the passivated surface. The formation of a single bimolecular layer of surfactant at the metal-water interface in step (b) permits formation of the desired passivating oxide layer in step (c).